Deleting Root Volumes Attached to Non-Existent Instances

Let's say you've got an OpenStack build you're getting ready to go live with. Assume also that you're performing some, ahem, robustness testing to see what breaks and prevent as many surprises as possible prior to going into production. OpenStack controller servers are being rebooted all over the shop and during this background chaos, punters are still trying to launch instances with vary degrees of success.

Once everything has settled down, you may find that some lucky punters have deleted the unsuccessful instances but the volumes have been left behind. This isn't initially obvious from the cinder CLI without cross checking with nova: